In recent years, a light emitting device or a display using a light emitting element formed with an organic material has been actively developed. The light emitting element is manufactured by sandwiching an organic compound between a pair of electrodes. Unlike a liquid crystal display device, the light emitting element itself emits light and does not require a light source such as backlight. In addition, the element itself is very thin. Therefore, the light emitting element is very advantageous to manufacture a thin lightweight display.
In a light emitting mechanism of the light emitting element, it is said that an electron injected from a cathode is recombined with a hole injected from an anode at a light emitting center of the organic compound to form a molecular exciton and the molecular exciton releases energy to emit light when returning to a ground state. A singlet excited state and a triplet excited state are known as excited states, and it is thought that light emission can be obtained through either of the excitation states.
An organic compound layer sandwiched between electrodes often has a laminated structure. A typical example of the laminated structure is functionally-separated laminated structure, for example, “hole transporting layer/light emitting layer/electron transporting layer”. By placing a layer of a highly hole transporting material on an electrode (serving as an anode) side and a layer of a highly electron transporting material on a cathode side, with a light emitting layer in which electrons are recombined with holes sandwiched therebetween, electrons and holes can be efficiently transported. Further, probability that electrons are recombined with holes can be increased. Since such a structure achieves very high emission efficiency, most of light emitting display devices which has currently been researched and developed employ this structure (for example, Reference 1: Chihaya Adachi et al., Japanese Journal of Applied Physics, Vol. 27, No. 2, 1988, pp. L269-L271).
In another structure, a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer, or a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer are sequentially laminated over an electrode serving as an anode. Each layer is formed with a material specific to each function. Note that a layer having two or more of these functions, for example, a layer having both functions of the light emitting layer and the electron transporting layer, may be used.
Although a layer containing an organic compound typically has a laminated structure as described above, it may be a single layer or be a mixed layer. In addition, the light emitting layer may be doped with a fluorescent pigment or the like.
Meanwhile, such a light emitting element has a problem in durability and heat resistance. Since such a light emitting element is formed by laminating an organic thin film containing an organic compound as described above, fragility of the thin film of the organic compound is considered as a cause for that.
On the other hand, there is an example of manufacturing a light emitting element using not an organic thin film but a layer in which an organic compound (a hole transporting compound, an electron transporting compound, or a light emitting compound) is dispersed in a skeleton composed of siloxane bonds (for example, Reference 2: Japanese Patent Laid-Open No. 2000-306669, and Reference 3: Tony Dantas de Morais et al., Advanced Materials, Vol. 11, No. 2, pp. 107-112 (1999)). Note that Reference 2 also reports that durability and heat resistance of the element are improved.
However, in the light emitting element disclosed in Reference 2 or 3, an organic compound is dispersed in a skeleton composed of insulating siloxane bonds. Therefore, current is hard to flow therethrough as compared with a conventional light emitting element.
Luminance of the light emitting element becomes high in proportion to the amount of current to flow therethrough. Thus, difficulty in flowing current through the light emitting element leads to an increase in voltage for obtaining a predetermined luminance (drive voltage), which results in higher power consumption.
In order to reduce short circuits in a light emitting element caused by dust or the like, it is effective to increase the thickness of the light emitting element. However, when the thickness of the light emitting element having such a structure as disclosed in Reference 2 or 3 is increased, drive voltage is markedly increased.